The present invention relates to the field of heat sink and heat spreader structures and, more particularly, to heat sink/spreader structures which utilize thermoelectric effects to more effectively dissipate thermal energy from Silicon-on-Insulator (SOI) based electronic devices.
Silicon-on-insulator (SOI) technology involves the formation of a thin semiconducting layer overlying a layer of insulating material. This structure reduces the power consumption and capacitance of the fabricated transistors thereby allowing faster switching speeds.
Many processes have been developed to yield this type of wafer material. Separation by implantation of oxygen (SIMOX) involves the implantation of a silicon substrate with a layer of oxygen which forms the oxide layer below the substrate surface. A thin silicon layer is then deposited on the surface which allows the fabrication of integrated circuit devices.
Bond and etch-back (BESOI) is used to manufacture relatively thick films of both oxide and silicon. Two silicon wafers, one with an oxide layer, are bonded together using van der Waals forces and annealing. Finally, one side is thinned for device fabrication.
Smart Cut Technology combines implantation and wafer-bonding technologies. A wafer is oxidized and then hydrogen is implanted through the oxide to form cavities at the implantation range. The wafer is then bonded at 500 C. causing a merging of the hydrogen cavities and a delamination of the wafer""s top section.
Although there are significant advantages associated with SOI technology, there are significant disadvantages as well. For example, poor heat removal from devices on an SOI substrate is a significant disadvantage. The oxide insulation layer has a markedly lower thermal conductivity than the thermal conductivity of conventional bulk silicon. For example, the thermal conductivity of silicon dioxide is about 1.4 W/m. degrees Celsius while the thermal conductivity of conventional bulk silicon is about 150 W/m. degrees Celsius.
To improve the thermal performance of an SOI-based electronics or integrated circuit device, heat sinks and heat spreaders are added either internally (bonded to the chip) or externally to the packages. However, the typical materials utilized exhibit a variety of shortcomings including: thermal expansion mismatch between the heat spreader and the chip, excessive weight, high cost, manufacturability issues and marginal thermal performance.
U.S. Pat. No. 6,166,411, granted to Buynoski on Dec. 26, 2000 discloses a method of fabricating SOI devices using metal substrates for heat removal. The metal substrate is coated with two oxide layers and is bonded (with heat and pressure) to an oxidized silicon wafer.
U.S. Pat. No. 6,121,661, granted to Assaderaghi, et al. on Sep. 19, 2000 discloses a silicon-on-insulator (SOI) structure in which trenches are etched from the active face of the silicon substrate, through the oxide layer and into the P-type substrate layer. These trenches are then filled with polysilicon, thereby creating xe2x80x9cplugsxe2x80x9d which help to dissipate heat from the circuitry (on the active face) to the substrate backside via the P-type substrate.
U.S. Pat. Nos. 5,793,107 and 6,080,608 granted to Nowak on Jun. 27, 2000 relates to polysilicon heat sink pillars formed on a silicon-on-insulator (SOI) wafer. Trenches, formed from the active face of the wafer through the oxide and to the P-type substrate, are filled with polysilicon and doped (N-type) to yield electrical isolation between the pillars and substrate.
Now, the field of Thermoelectricity relates to the thermodynamic effects of temperature differentials, electric potential gradients and current flow in single and multiple dissimilar electrical conductors or semiconductors. There are basically three effects which comprise this field including: the Seebeck Effect, the Peltier Effect and the Thomson Effect.
In 1821, Seebeck found that when two dissimilar conducting or semiconducting materials are joined to each other at both ends and if there is a temperature differential between these two ends, an EMF, or voltage, will be established within the two materials. This effect is called the Seebeck Effect. The effect arises because the presence of a temperature gradient in a material causes a carrier-concentration gradient and an electric field is established which causes the net flow of charge carriers when the conductors are joined into a closed electrical circuit.
In 1834, Peltier observed that heat was either liberated or absorbed at the junction of two dissimilar conductors or semiconductors when an electric current was passed through the junction. Measurements established that the rate of absorption or liberation of heat at the junction was directly proportional to the electric current. The effect arises because the potential energy of the charge carriers is in general different in the two conductors and also because the scattering mechanisms that govern the equilibrium between the charge carriers and the crystal lattice differ in the two conductors. Therefore, in order to maintain a conservation of energy as well as a conservation of charge when charge carriers move across the junction, energy must be interchanged with the surroundings of the junction. As in the case of the Seebeck Effect, the Peltier Effect cannot be ascribed to either material alone but rather is a consequence of the junction.
In 1857, Thomson found that an energy interchange with the surroundings took place throughout the bulk of a conductor if an electric current was allowed to flow while a temperature gradient existed in the conductor. The rate of energy absorbed or liberated per unit length was proportional to the product of the electric current and the temperature gradient. The reasons for the existence of the Thomson Effect are essentially the same as those that cause the Peltier Effect. However, the difference in the potential energy of the charge carriers and in the scattering mechanisms are the consequences of the temperature gradient and not of the inhomogeneities in the conductor.
Additionally, charge carriers which flow (induced by a voltage) from one region of any conductive or semiconductive material to another carry with them small quantities of heat energy. If the carriers originally at one temperature in the conductor are displaced to cooler surroundings, they must discharge their excess kinetic energy by collisions with the lattice, thereby maintaining a conservation of energy. This process assists the normal thermal conduction of heat energy in the conductor, which would occur in the absence of charge carrier (electric current) flow.
By combining the electric charge induced (active) heat transfer mechanisms created by the thermoelectric effects with the thermal conduction/radiation (passive) heat transfer mechanisms of typical heat sink/heat spreader structures, a more effective heat management structure is produced.
Accordingly, it is the overall object of the present invention to develop and construct heat dissipating silicon-on-insulator (SOI) structures which utilize thermoelectric effects in order to more effectively transfer thermal energy from electronic circuitry fabricated from these structures.
One object of the present invention to provide a heat dissipating SOI structure in which the silicon substrate itself is part of a thermoelectric couple, which may have an external electric potential applied.
An additional object of the present invention provides a heat dissipating SOI structure comprising a thermoelectric couple with elements connected together at both ends. When subjected to a temperature gradient, an EMF and corresponding current is established within the couple resulting in the absorption and liberation of heat at these junctions without the need for external electrical power.
Another object of the present invention is to provide a heat dissipating SOI structure, comprising of simply an electrically conductive or semiconductive material, wherein the material has an external electric potential applied in order to induce multiple heat transfer effects through the structure.
In another object of the present invention, the thermoelectric couple or conductive material of the heat dissipating SOI structure is in electrical series with an external electric load such as an electronic component or other thermoelectric device.
In yet another object of the present invention, the thermoelectric couple or conductive material of the heat dissipating SOI structure is utilized as a resistive load for an electronic component or power supply circuit in order to reduce electrical power consumption of the system.
Still, another object of the present invention is to provide a heat dissipating SOI structure whereby the thermoelement couple, when subjected to a temperature gradient, provides electrical power to an external load.
A further object of the present invention is to provide unique methods of delivering electrical power to each thermoelement, conductor or substrate.
Another object of the present invention is to provide a heat dissipating SOI structure comprised of multiple thermoelectric couples, in a planar configuration.
In yet another object of the present invention, a cascaded, or multistage xe2x80x9cplanarxe2x80x9d thermoelectric device structure is illustrated, wherein each successive stage is added to the horizontal plane. The heat absorbing junctions of the stage interfacing the heat source are located in the center of the structure and the heat rejecting junctions of the heat rejecting stage are located within the perimeter of the structure.
A further object of the present invention includes a single or multistage heat sink/spreader, each stage consisting of at least one thermoelement couple, in which all thermoelements are formed within the SOI substrate through selecting doping techniques. Various electrical isolation techniques are additionally disclosed.
An additional object is to reduce the coefficient of thermal expansion mismatch between integrated heat spreaders and SOI structures.
Lastly, it is an object of the present invention to combine all of these unique design aspects and individual fabrication techniques into effective and manufacturable heat dissipating SOI structures.